1. Field of the Invention
The invention relates to a Steckel hot rolling mill with at least one reversing roll stand, and coilers with torque-controlled drives positioned upstream and downstream of the roll stand.
2. Description of the Related Art
Steckel hot rolling mills of this type have torque-controlled coilers wherein, however, the control for achieving constant torques during the operation, particularly during rolling of hot-rolled strips, leads to insufficient rolling results. In such coilers with their partially large, inert masses, tension variations occur in the strip during the acceleration or deceleration phases at the strip beginning or the strip end or in the case of mass flow defects, wherein the variations cannot be regulated by the torque control, so that the known plants are only permitted to be operated with limited deceleration or acceleration. Such a limited acceleration or deceleration results in longer reversing times, lower rolling speeds and, thus, colder strip beginnings or strip ends which, in turn, require higher rolling forces. Substantial changes of the process variables, such as temperature, rolling force, together with loss of tension due to coiler unbalances and mass flow changes, lead to losses of quality and stability, such as, for example, out-of-center travel of the strip.
Therefore, the invention is based on the object of further developing a Steckel hot rolling mill of the above type in such a way that changes of the process variables due to changes of tension and/or mass flow can be counteracted in an optimum manner and that it is especially possible to roll thin hot-rolled strip with uniform, high quality.
To this end, it is proposed that a looper each is provided between the coilers and the reversing stand, wherein each looper supplies actual values for a tension control and a mass flow control. Consequently, certain tensions can be adjusted on each side of the reversing roll stand through the two loopers. If mass flow changes occur at the strip entry side or the strip exit side which are characterized essentially by changes of the strip speed, a mass flow control is effected by controlling the strip coiling speed or rate of rotation of the coiler for achieving an adjustment of the mass flow to a desired value.
It is an advantage if the loopers have a torque control effecting a constant strip tension, wherein a correction value is added to the torque control in dependence on the looper angle. It is further advantageous if a mass flow computer determines in dependence on the looper angle speed correction values for a control of the rate of rotation of the coiler. The mass flow control added to the tension control makes it possible to regulate high-frequency defects.
If the coilers are equipped with preliminary mass flow control and/or a preliminary mass flow regulation, it is ensured that changes, for example, of the desired thickness values or changes in the roll stand geometry, can already be regulated prior to the occurrence of tension or mass flow changes which would be recognized by the loopers.
Another advantage is to be seen in the fact that the coiler shafts are provided with angle transmitters which make it possible to determine deviations of the coiling or uncoiling speeds which are supplied to the tension regulators of the strip as preliminary control variables. This makes it possible that tension or mass flow changes resulting from eccentricities of the coilers can be taken into consideration during a preliminary control for regulating the loopers, without having to have the errors caused by the eccentricity recognized by the looper and only then having to regulate out these errors subsequently.
Essential for the operation of the Steckel hot rolling mills according to the present invention is the fact that low-inertia mass-optimized loopers which follow high-frequency changes are used. By using a special geometry and components of the loopers which are optimized with respect to their mass, it is achieved that these loopers can follow very rapid changes in tension or mass flow so that the errors measured in this manner can be counteracted by the corresponding control circuits.